Field of the Disclosure
This disclosure is directed to high density slurries comprising encapsulated sulfur and/or chalogen particles, carbon nanofibers and activated carbon black. The slurry is suitable for use in forming the active material of an electrode. This disclosure is also directed to a cathode formed from the dried slurry as well as an electrochemical cell or battery containing the cathode.
Discussion of the Background
Future electric vehicles and portable electronics will demand the advent of super-batteries with higher energy densities than what is currently offered by lithium ion. The goal of battery research is to couple a metal anode such as lithium or magnesium with a high capacity conversion cathode such as sulfur or oxygen without sacrificing cycle life and rate capability. While sulfur is economical, highly abundant and has a charge capacity an order of magnitude higher than insertion lithium ion cathodes, it is electrically insulating and it suffers from mass loss during cycling due to reduction intermediates that are highly soluble.
While sulfur has been an attractive cathode for more than fifty years, its viability has been hindered by two fundamental challenges which need to be resolved. The first challenge is to enhance the conductivity of elemental sulfur. Unlike commercial lithium ion cathodes (i.e. LiCoO2) which possess a high electronic conductivity and do not require significant addition of conductive additives, sulfur is a good insulator. Sulfur is approximately 1 billion times less conductive than LiCoO2. The second challenge is to control the diffusion of polysulfide intermediates formed during cycling. During discharge, sulfur reduces in a stepwise manner by forming a series of polysulfide intermediates which are ionic in nature and solvate easily in the electrolyte. This causes mass loss of active material upon cycling. Even today, while approaches mitigate these fundamental challenges of low conductivity and dissolution of polysulfides, they also diminish the superior charge capacity of sulfur. One problem deriving from the insulating nature of sulfur is the need for high loadings of conductive additives to improve the overall electronic conductivity. This results in low sulfur content in the cathode. A second problem is the slow rate of operation due to the low electronic conductivity of sulfur and the low ionic conductivity of the reduced product, Li2S. A third problem is the diffusion of ionic polysulfides which limits cycle life due to anode passivation and mass loss from the cathode.
Extensive research efforts have been devoted to developing methods to enhance the conductivity of elemental sulfur and to control the diffusion of polysulfide intermediates formed during cycling. Researchers have studied using sulfur infusion into conductive hosts or polymer coatings of sulfur composites to address these problems. Since the benefit of infusing sulfur into ordered mesoporous carbon was demonstrated, various micro/nano carbon hosts including spheres, nanofibers, graphene oxide and carbon paper have been investigated as conductive hosts to contain the sulfur active material (Nazar et al. Nature Materials, 20089, 8, 500-506). A microporous carbon interlayer with pore sizes matching the dimensions of the polysulfide ions has recently been demonstrated (Manthiram et al. Nature Communications, 2012, 3, 1166). In addition, infused sulfur into metal organic frameworks (MOF) with the goal of benefiting from interactions between the polysulfides and the MOF oxide surface has been shown (Tarascon et al. Journal of the American Chemical Society, 2011, 133, 16154-16160). While these approaches improve the conductivity of the sulfur cathode, they are still plagued by diffusion of polysulfides out of the host pores which limits cycle life. Furthermore, the overall sulfur loading in the electrode is low (˜<50% sulfur ratio with an areal loading of sulfur around 1 mg S/cm2) which is too low to compete with the energy density of commercial lithium ion batteries.
In view of the forgoing, there is a need to provide batteries having significantly increased energy density and thus the generation of high capacity cathode materials is necessary. Thus, one object of the present disclosure is to provide a high density slurry comprising elemental sulfur and conductive materials which is suitable for use in the formation of the active material of an electrode. A second object of the present disclosure is to provide a method for forming the high density slurry. A third object of the present disclosure is to provide an electrode comprising the dried high density slurry as an active material which is suitable for use in a battery having a significantly increased capacity and high cycle lifetime. A fourth object of the present disclosure is to provide a battery comprising the electrode having a sufficiently high capacity and a sufficiently high cycle lifetime to be a power source for an electric vehicle.